Lithium secondary battery, with its high operating voltage and energy density and superior output characteristics, is widely used as an energy source for the electronic products or electrically-driven apparatuses.
The lithium secondary battery includes a cell assembly and an electrolyte, and a package that sealingly accommodates the cell assembly and the electrolyte therein and that has an electrode terminal formed thereon.
The cell assembly includes a plurality of unit cells connected in parallel, and each of the unit cells includes at least a positive electrode plate and a negative electrode plate, and a separator interposed therebetween.
The positive electrode plate and the negative electrode plate each includes a positive electrode active material coating layer and a negative electrode active material coating layer where lithium ions can be intercalated or de-intercalated. The lithium metal oxide and graphite are the representative examples that are used as the positive electrode active material and the negative electrode active material.
Traditionally, the lithium secondary battery has been mainly used in a handheld terminal such as mobile phone, laptop computer, video camera, electric tools, and so on.
However, the recent concerns over depletion of petroleum have prompted extended use of the lithium secondary battery to the field of electrically-driven vehicles. The electrically-driven vehicles, or vehicles driven by electrical energy, include electric vehicles (EV), hybrid electric vehicles (HEV), plugin HEV (PHEV), and so on.
In order to be adapted for use in the electrically-driven vehicles, the lithium secondary battery is required to have high outputs and energy density. Only by that way it is possible for the battery to support excellent running performance and also increase the driving distance of the electrically-driven vehicle.
The output from the lithium secondary battery can be enhanced by enhancing the positive electrode active material.
For example, Korean Patent Publication No. 2014-0018542 discloses that use of a blended positive electrode material including a first active material having good outputs in high SOC range, mixed at a particle level with a second active material having good outputs in low SOC range leads into enhanced outputs of the lithium secondary battery.
However, blending active materials at a particle level can deteriorate microstructure of the active material coating layer due to differences in the physical characteristics of the respective active materials.
For example, when the active materials forming a blended positive electrode material have different coefficients of expansion or mechanical strengths from one another, this may cause cracks within particles or even crush the particles during the process of coating the blended positive electrode material on the positive electrode plate.
Further, in order to obtain an effect of increased output by use of the blended positive electrode material, it is necessary to optimize the particle shape or size distribution of each of the active material particles, but the process takes a considerable amount of time and cost.
Meanwhile, Korean Patent Publication No. 2014-0092554 discloses improving an output of a battery pack by constructing mixed cells by connecting in parallel the heterogenous cells with different electric characteristics from each other, and then constructing a battery pack by repeatedly connecting the mixed cells in series.
This mixed cells have a structure in which two independently-packed cells are connected in parallel. In such structure, when one of the cells has a relatively faster degradation, the service life of the battery pack would be shortened rapidly.
Cell degradation can happen mainly due to the loss of lithium ions contained in the electrolyte by the side reaction, or due to the structural decay of the positive electrode active material or the negative electrode active material. However, when two cells are independently packed, it is difficult to have electrochemical reactions between the cells to compensate for the degradation of either one of the cells.